The major objective of this proposal is to study the cellular and molecular events associated with the differentiation of normal and malignant human lymphoid progenitor cells (LPC). The study will emphasize the clonal expansion of progenitor cells in non-T, non-B acute lymphoblastic leukemia (ALL) and their nonmalignant counterparts present in bone marrow (BM). Emnphasis will be placed on the use of 2 monoclonal antibodies recently produced in our laboratory, designated BA-1 and BA-2, that appear to bind to normal and malignant LPC at various stages of B cell development. Monoclonal antibodies against the common acute lymphoblastic leukemia antigen (CALLA) and HLA-DR (human Ia-like) antigens will also be used. Three specific aims will be pursued. 1) A large number of lymphohematopoietic cell sources will be analyzed to determine the relationship between the aforementioned monoclonal antibodies, and 2 intracellular markers of LPC, i.e., terminal deoxynucleotidyl transferase (TdT) and cytoplasmic IgM (CIgM). The cell populations will be assayed by single or double fluorochrome staining using fluorescent microscopy and flow cytofluorimetry. The cell sources will include fetal hematopoietic tissues, normal BM, "regenerating" BM, Epstein-Barr virus transformed LPC cell lines, and long term bone marrow cultures ("Dexter cultures"). 2) The phorbol ester TPA will be used to study the induction of differentiation in normal and malignant LPC. TPA-induced changes will be analyzed using immunofluorescence and biosynthetic labeling followed by SDS-PAGE. We will attempt to determine if there is a sequential (differentiation-linked?) expression of cell surface structures recognized by monoclonal antibodies, and their relationship to the intracellular markers of LPC, i.e., TdT and CIgM. 3) We will continue to produce monoclonal antibodies recognizing cell surface structures on LPC that we hypothesize are part of the B lymphocyte lineage. Such cells appear to have a bone marrow origin beyond fetal life, but their early differentiation pathways and traffic patterns are poorly understood. It is possible that significant perturbations of these early events are central to the pathogenesis and etiology of ALL and various immunodeficiencies. In this grant, we propose to conduct fundamental studies on normal and malignant LPC to enhance our understanding of these complex events.